In a typical cellular communications system, wireless user equipment units (UEs), for example, mobile phones or other types of mobile terminals, communicate via a radio access network with one or more core networks. A radio access network covers a geographical area which is divided into cells, with each cell area being served by a radio base station. Several base stations are connected, typically via land lines, to a control node known as a radio network controller (RNC). Such a control node supervises and coordinates various activities of the several radio base stations which are connected to it. The radio network controllers are typically connected to one or more core networks. One example of a radio access network is the Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN). The UMTS is a third generation (3G) system and UTRAN is essentially a radio access network providing wideband code division multiple access (WCDMA) to user equipment units. Fourth generation systems are evolving towards a broadband and mobile system. The 3rd Generation Partnership Project has proposed a Long Term Evolution (LTE) solution, namely, an Evolved Universal Mobile Telecommunication System Territorial Radio Access Network, (E-UTRAN), for a mobile access network. The 3GPP LTE standard aims to improve the Universal Mobile Telecommunications System (UMTS) terrestrial radio access mobile phone standard to cope with future requirements. The 3GPP LTE technical specification is described in a set of reference documents including LTE; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2; 3GPP TS 36.300 version 9.3.0 Release 9 (2010 April). In 3GPP LTE (E-UTR A and E-UTRAN) terminology, a radio base station is called an “eNode-B” (eNB) and a mobile terminal or device is called a “user equipment” (UE).
In many radio access networks the radio base station is a concentrated node with most of its components being located at a concentrated site. However, a radio base station can also be configured with a more distributed architecture. For example, a distributed radio base station can take the form of one or more radio equipment (RE) portions that are linked to a radio equipment control (REC) portion over an internal interface. One example of an internal interface of a radio base station which links a radio equipment portion of the radio base station to a radio equipment control portion of the base station is the Common Public Radio Interface (CPRI). The Common Public Radio Interface (CPRI) is described in Common Public Radio Interface (CPRI) Interface Specification Version 4.1 (18 Feb. 2009) and in Version 5.0 (Sep. 21, 2011).
The Common Public Radio Interface (CPRI) is an industry co-operation aimed at defining a publicly available specification for the key internal interface of radio base stations between radio equipment control (REC) and radio equipment (RE), thereby allowing base station manufacturers to share a common protocol and more easily adapt platforms from one customer to another. In essence, a radio base station is decomposed into two separate blocks, known as REC and RE. The REC provides access to a UMTS network, for example, via the lub interface, whereas the RE serves as the air interface to user equipment, known as Uu in a UMTS network. The REC generally comprises the radio functions of the digital baseband domain, whereas the RE contains analogue radio frequency functions. This functional split between the REC and RE allows the RE to be positioned close to an associated antenna. This reduces the distance which the associated signals have to travel before they are received by the RE, thereby negating the need for tower-mounted amplifiers and antenna system controllers. The link between the RE and REC is generally optical, allowing the link length to be much greater when compared with wired coaxial systems. Therefore, the distance between the RE and RRC can be around 10 Km, thereby increasing the flexibility of deployment of RE's within the network when utilising CPRI.
An REC is generally configured to comprise the radio functions of the digital baseband domain. In order to support LTE for example, in the downlink (that is from the REC to the RE), the REC must provide such functions as modulation, channel coding and interleaving, inverse Fourier transform processing and frame and slot generation. In the uplink (that is from the RE to the REC), the REC must provide such functions as Fourier transform processing, demodulation, channel decoding and the interleaving and signal distribution. An RE is generally configured to comprise analog and radio frequency functions. In order to support LTE for example, the RE, in the downlink, performs operations such as digital to analog conversion, up conversion and carrier multiplexing. In the uplink the RE performs operations such as down conversion, carrier demultiplexing and analogue to digital conversion. Many of the functions which an REC has to perform may be realised by a proprietary digital signal processing device. Two examples of DSP devices which support the CPRI are the Freescale Semiconductor B4860 and the Freescale MSC 8157 Broadband Wireless, Access Six Core DSP which is described in Freescale Semiconductor Data Sheet MSC8157E, November/2011. Typically, such devices employ a Direct Memory Access (DMA) operation to transfer data received over the CPRI link to system memory and also to fetch data from system memory for transmission over the CPRI link to the RE.
The functional split between the REC and RE is done in such a way that a generic interface, CPRI, based on In-Phase and Quadrature (IQ) data can be defined. Several IQ data flows can be sent over one physical link with each data flow reflecting the data of one antenna for one carrier, the so-called antenna carrier “AxC.” Several AxC's having the same sampling rate may be aggregated into an “AxC Group.” IQ data samples of different antennas along with control data are multiplexed onto a transmission line. The CPRI has a basic frame structure for carrying a control word and an IQ data block. An “AxC container” is defined as sub-part of the IQ data block of one basic frame.
LTE systems may support several different sampling rates (as detailed in the above-referenced 5GPP TS), notably 5 MHz, 10 MHz, 15 MHz and 20 MHz. Hence, in order to meet LTE requirements, a CPRI link must be able to support a multi-bandwidth connection between REC and RE. A known method of supporting two different sampling rates over the CPRI link requires the use of a dual bandwidth DMA process (employing two channels, one for each sampling rate). Typically, for a 5 MHz AxC there are two IQ samples each basic frame and for a 10 MHz AxC there are four IQ samples each basic frame. Typically, the data samples are arranged using packed positioning, for example, a basic frame may have the form IQ0—0, IQ1—0, IQ1—1, 0, . . . 0. Two Fast Fourier transform (FFT) processors for the uplink (and two inverse Fast Fourier transform (iFFT) processors for the downlink) are required, one for each sampling rate. Each FFT/iFFT processor acts on a different number of samples, a smaller number for the lower sampling rate and a greater number for the higher sampling rate. Dual and multiple DMA processes tend to increase system complexity and cost.